The present invention relates to a method for controlling an inverter by means of space vector modulation, in particular for controlling an electrical machine, wherein the inverter has a plurality of controllable switches and a corresponding plurality of freewheeling diodes, and which is designed to provide a polyphase electric current and a polyphase electric voltage in the form of space vectors, in particular in order to supply the electrical machine with electric current in a polyphase fashion, wherein the inverter is controlled in order to set up a plurality of successive different switching states of the switches, wherein, by means of two switching states, the inverter is switched into a switching state which switches to zero potential.
The present invention also relates to a device for controlling an inverter which has a plurality of controllable power switches and a corresponding plurality of freewheeling diodes which are connected to provide a polyphase electric current and a polyphase electric voltage in the form of space vectors, in particular in order to supply the electrical machine with electric current in a polyphase fashion, having a control device which is designed to control the inverter such that the inverter assumes a plurality of successive different switching states of the switches, and wherein the inverter can assume two switching states which switch to zero potential.
The present invention also relates to a motor-vehicle drivetrain having at least one electrical machine for providing drive power, an inverter for controlling the electrical machine and having a device of the type described above for controlling the inverter.
Various control methods are known in the technical field of three-phase loads in general and of three-phase electrical machines in particular. In this case, the method of space vector modulation is currently usually preferable for controlling the three-phase load. In the case of this control method, a space vector is formed by consecutive setting of eight basic voltage phasors. In order to provide the string voltage, the basic voltage phasors are switched in a pulse-width-modulated manner, with the result that a corresponding control voltage is generated.
In the case of the known control methods, the electrical loads are controlled by means of an inverter with power semiconductor switches. The eight consecutive basic voltage phasors for generating the voltage space vector are set by virtue of particular power semiconductor switches of the inverter being alternately switched on and off. In the case of very low rotation speeds of the space vector or, provided the three-phase load is an electrical machine, in the case of low speeds of the controlled electrical machines, individual ones of the power semiconductor switches are switched very often or for a very long time and are thus thermally loaded with an electric current which flows for a very long time or very often. Therefore, the power semiconductor switches must be configured for very long switch-on times and for very large currents, which results in the inverter being generally elaborate in terms of technology.
In order to respond to an overload, in particular a thermal overload, of the power semiconductor switches, it is proposed in WO 2010/000548 A2, for example, to omit one of two switching states which switch to zero potential in particular pulse-width-modulation periods in order to reduce the switching losses of the power semiconductor switches.
Since the loading, in particular the thermal loading, of individual power semiconductor switches of the inverter is dependent on a phase angle of the provided current space vector or individual ones of the power semiconductor switches of the inverter are differently loaded for particular phase angles of the provided current space vector, it is proposed in DE 10393516 T1, for example, to use a particular zero vector in particular angular regions of the provided current space vector in order to reduce the switching losses of the power semiconductor switches.
It is disadvantageous here that, in certain control situations, particularly in the case of very low rotation speeds of the space vector, the losses occurring can heavily thermally load individual ones of the power semiconductor switches and/or individual ones of the freewheeling diodes, as a result of which overloading of the power semiconductor switches and/or the freewheeling diodes cannot be avoided.